It turns out the tracking and timing of subway trains is harder than one might think.
Michael M. Grynbaum writes, "The city's subway tracks are equipped with signals that follow trains through the system. That is why trains do not run into one another, or sometimes speed up or slow down to conform to schedules.
But it is hard to harness the information from those signals, and harder still to convert it into an approximate arrival time that can be displayed for passengers. Signals are not routed to a central processing center, so controllers cannot see an entire route at once, and GPS and wireless signals do not travel well underground."